Go to Inbound > Inbound Orders and select the filter Partially Received.
Enter the quantity received.
Enter a new vendor bill number for this specific shipment.
Hit Receive.
If you check Vendors > View Bills/Credit Memos, under the filter pending, you can see this new bill. If you finalize the bill by pressing the Create Bill button in the top right corner, you will now see two separate bills (with different bill numbers) tied to that single Vendor PO.